This invention relates to a conductive adhesive especially useful in biomedical electrodes used to establish an electrical connection between the skin of the human anatomy and an electromedical apparatus, such as a high impedance electromyograph, electrocardiograph, electrical neurostimulator for pain relief, and the like. More particularly it relates to a conductive adhesive for use in so-called "dry" bioelectrodes which do not require the use of messy creams or gels to enhance conductivity between the skin and the electrode plate.
A variety of disposable biomedical electrodes are known in the art. Generally, they comprise a metallic electrode plate adapted for connection to a lead wire which is, in turn, attached to the electromedical apparatus. Typically, a paste, cream, or gel which contains ionic material is relied upon to conduct the electric current and improve the electrical connection between the skin of the patient and the electrode plate. An adhesive tape is commonly used to adhere the entire apparatus to the skin. Examples of electrodes of this general type are described in U.S. Pat. Nos. 3,587,565 and 3,805,769.
The conductive pastes, creams, or gels used in these prior art biomedical electrodes are unpleasant to use, sloppy, and often irritating to the skin particularly when the skin is cleaned and abraded prior to application of the electrode. Since these electrodes all contain water as the major ingredient to solvate the ions present and function as a medium through which the solvated ions migrate, they require elaborate packaging to prevent loss of water prior to use. Furthermore, they leave a residue on the skin after removal of the electrode which requires cleanup. A further disadvantage of the electrodes of the conductive paste, cream, and gel types is that they may develop an over-potential in defibrillation procedures unless the surface of the electrode plate is of expensive silver/silver chloride.
To overcome many of the problems associated with so called "wet" electrodes, biomedical electrodes having an integrally formed metal snap connector have been proposed which utilize "dry" conductive material. U.S. Pat. Nos. 4,008,721 and 3,911,906 disclose biomedical electrodes utilizing adhesives impregnated with conductive particles. These adhesives serve the dual purpose of enhancing conductivity with the skin and securing the electrode to the skin. Although avoiding the sloppiness and packaging problems associated with gels and pastes, such electrodes generally do not provide satisfactory electrical connection to the skin because the presence of the conductive filler results in a high signal-to-noise ratio and is deleterious to adhesion. Generally, the use of nonhomogeneous conductive formulations in bioelectrodes has been found to give rise to noisy electrical signals. It is speculated that dispersed metal or salt particles in a binder matrix form a discontinuous, electrically conductive path which develops random, nonuniform electrical fields between particles which cause noise.
Another biomedical electrode used for transcutaneous electrical neural stimulation (TENS) disclosed in U.S. Pat. No. 4,125,110 utilizes a natural polymer, namely, gum karaya, for securing the eloctrode to skin. Gum karaya is a complex polysaccharide combined with certain metallic cations, such as sodium, potassium, calcium, or magnesium. The gum does not dissolve but swells in water to a paste-like gel (Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 10, 1966). Because natural polymers originate in nature where soil and climatic conditions are variable, and the conditions under which they are collected and processed are variable, there is a great inconsistency in the physical and chemical properties of natural polymers and in the amount of impurities present. Such inconsistency leads to variations in the electrical performance of biomedical electrodes made from natural polymers. This variation in electrical performance cannot be tolerated in biomedical electrodes where consistent electrical properties are important for signal reception. Furthermore, the natural polymers are undesirable because they generally support undesirable microbial growth and have the potential for creating adverse skin sensitivites including allergenic and antigenic reactions (Merck Index, 8th Edition, 1969, page 598).
Other "dry" electrodes are also known. U.S. Pat. No. 3,812,861 teaches a grounding electrode consisting of a flexible sheet of paperboard coated on both sides with a conductive foil joined electrically together and a means for tightening the device around a limb. Such electrodes having a metal-to-tissue interface are undesirable because of the bio-incompatability of most metals and the difficulty of obtaining adequate conformability with the body surface. U.S. Pat. No. 4,066,708 has addressed this metal bio-incompatability by placing a conductive layer on the skin contacting surface of the metal. The disclosed conductive formulations consist of suspension polymers which require numerous process steps, e.g. removal of solvents, which would tend to substantially increase the cost associated with the manufacturing of the electrode.
Another conductive adhesive has been called to applicant's attention which is a synthetic hydrophilic polymer containing at least 5 mole percent of monomer units containing salts of a carboxylic acid, see "Biomedical Electrode," U.S. patent application Ser. No. 940,734 filed Sept. 8, 1978 and owned by the same assignee as the present application.
The biomedical electrode and the conductive adhesive according to the present invention offer several advantages over the previously described biomedical electrodes. First, the present electrode by virtue of its dry conductive material eliminates messy gels, creams or pastes. Secondly, the conductive material is formed by an essentially solventless process in substantially its final form. This can be done in situ on a releasable surface for a later transfer to the electrode plate or directly on the electrode plate which eliminates the need for an additional transfer step. Neither of the aforementioned formation methods require process steps which are associated with the removal of solvent from suspension polymer formulations. Thirdly, the resultant conductive materials which is formed has homogeneously dispersed therein free ions which minimize the potential for the formulation of nonuniform fields which are associated with dispersed metal or salt particles in binder matrices.